There is no such sport as 'Rugby' which is merely a style of football, there are two distinct sports called 'Rugby Union' and 'Rugby League', which whilst they have a common heritage and some common skills, are very different in many ways, have different clubs, national teams, laws and governing bodies etc. They have been separate sports for nearly 120 years.

Most people who talk about Rugby are actually talking about Rugby Union, but it would be presumptive to have Rugby as a synonym for Rugby Union, as League is a popular sport in many countries, particularly Australia, where it is more popular than the other.

I do think that most of the questions tagged Rugby so far could be retagged as Rugby Union, and a separate Rugby League tag could be made.

I guess we could just add the -Union -League tags as appropriate when people tag questions with just Rugby when the question is actually about a specific code. I don't have the reputation yet to retag, but I'll get there at some point.
– iandotkellyJun 1 '12 at 13:35

I am not much of an expert in rugby but I have colleagues that play(ed) in teams, one of which have played for the national side in youth teams. Based on the chats I've had with them, I'd say the distinction is not large enough (like rugby vs aussie-rules football) to require different tags as of now, seeing as there are not that many rugby related questions at this point. That's my two cents...

Btw: great meta question, IMHO! :)

Edit: I just had a small chat with my colleagues and got insulted based on the fact that I considered the Union and the League similar enough to be considered two flavors of the same sport. :) Apparently for someone who's a bit more involved and informed there's a huge difference...